In the field of integrated circuit processing, it is essential to isolate one transistor from a neighboring transistor or other component. The art currently uses a trench isolation in which a trench is etched into the electrically active silicon and filled with oxide, or LOCOS isolation in which thermal oxide is grown downwardly into the silicon. Variations on these schemes are well known in the art, all commonly having the factor that there is an insulator, usually SiO.sub.2, of a thickness sufficient to prevent voltage on an interconnection line above the insulator from inverting the silicon beneath the insulator and thereby creating a parasitic channel; and that all or part of the isolation is recessed to provide for smaller step height for the interconnection. Throughout the years, the art has tended to smooth topographical features in the isolation while maintaining protection against parasitic channel formation.
Before the introduction of LOCOS, when design rules were above 5 .mu.m, a blanket oxide was grown over the wafer before any of the elements of the transistors were fabricated. Apertures were etched into the blanket insulation to hold the transistors. In this case, topographical features were severe, which caused significant reduction in yield as design ground rules became smaller and the step to be traversed by interconnects became sharper. One distinguishing feature of this old prior art work compared with more recent prior art work was that there was a direct line between adjacent transistors along the surface of the single crystal silicon. Since the minimum distance between transistors was on the order of 5 .mu.m, there was considerable resistance on this path that is not present in modern submicron technology.
A number of problems have also become apparent in the case of trench isolations, which are preferred to the old blanket approach because of their planar surfaces and because of the elimination of the "bird's beak" associated with LOCOS isolations, thereby permitting a shorter, smaller transverse dimension than LOCOS isolations. The approaches in the art have attempted to reduce step size by recessing the insulator into the single crystal area, with the result that there was an increased risk of creating defects in the single crystal material in the process of forming the isolation. The art has long sought a method of isolation in which the smallest transverse distance can be obtained without compromising the effectiveness of the isolation.